There is an argument how many walls does a Rshus Harabim need to have to make it a Rshus Hayachid (Deoraisa). R' Yehuda says that one needs two walls, while the Chachamim said one needs three.

The Gemara (there) says that a Sratya (a through street going through the city) and a Playta (a city square) are Rshus Harabim. However, according to R' Yehuda, all Sratyas and Platyas are surrounded by walls on two sides.

I may be misunderstanding your question. Who says that the black areas are necessarily walls? Suppose you just have a row of unattached houses on one or both sides of the sratya.
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AlexFeb 7 '12 at 4:06

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I had to read this question multiple times to get what the question was. Could you spell it out more explicitly, please?
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Isaac MosesFeb 7 '12 at 4:07

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I'm with Alex. Also, can you give citations in Eruvin for the machloket RY and Chachamim and the din of sratya and platya?
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Double AA♦Feb 7 '12 at 4:27

@Alex it says that if the gates of Yerushalayim would not have been locked at night it would have been a Reshus Harabim. Yerushalayim has walls.
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Shmuel BrinFeb 7 '12 at 4:32

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@msh210 the gemora cited here is from mesechet Shabbat, not Eruvin. Curious if this really has to do with eruvin at all. :)
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aviFeb 7 '12 at 18:57

3 Answers
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I'm going run with the braisa working in the chachamim's world. The seratia is a reshus harabim, not a reshus hayachid. What then is a reshus hayachid? An enclosed pit, a fenced in area.

Rabbi Yehuda would hold that if you have courtyards on either side of what would have been a reshus harabbim, that is a reshus hayachid. If Rabbi Yehuda would have a braisa, he would have said that seratia is private domain, and a public domain would not contain walls.

(Although it is possible that Rabbi Yehuda would say the seratia areas adjacent to alleyways are considered public since the road is still mefulash. This would answer the outstanding question of why the chachamim's braisa had to say zu hi to reject Rabbi Yehuda and could not rely on the halacha that seratia is reshus harabbim.)

R. Yehuda it talking about how to divide up a Rishut Harabimb into smaller areas, so that they no longer qualify as a Rishut HRabamim. Just because 2 walls are needed, does not mean that you can stick two walls any distance appart and say that this is now a Rishut Hayachid.

The situation of a Town square and the large street, will always be a rishut HaRabim. If for some reason you stick up two walls in some place, then R. Yehuda argues that this place is nolonger a large street and a town square but rather it's now 2 seperate places. The Rishut Hayachid and the rest of the place which is still rishut Harabim.

Avi, the gemara there quotes R"Y's case s having 2 houses on opposite sides of a reshus harabim. The required lechi's are (acc. to rashi) only for heker and the reshus hayachid should exist without them. For example, one would be chayav if he threw somehing there from a reshus harabim even without the lechi's. @ShmuelBrill's question is how can this reshus harabim coexist with a reshus hayachid.
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YDKFeb 7 '12 at 19:52